This invention relates to compression release engine braking systems for turbocharged internal combustion engines, and more particularly to such systems in which increased engine braking horsepower can be provided at low and moderate engine speeds without unacceptably high stress on components of the engine and engine brake at high engine speeds.
Compression release engine brakes for internal combustion engines are well known as shown, for example, by Cummins U.S. Pat. No. 3,220,392. The purpose of such brakes is to convert the associated engine from a power source to a power sink when braking is desired. Such power sinking may be helpful to assist the wheel brakes of a vehicle (e.g., a tractor trailer truck) propelled by the engine, thereby prolonging the life of the wheel brakes and increasing the safety of operation of the vehicle. When engine braking is desired, the flow of fuel to the engine is cut off and the engine brake is turned on. Turning on the engine brake causes it to open at least one exhaust valve in at least one engine cylinder (preferably several or all engine cylinders) each time the piston in that cylinder nears top dead center of its compression stroke. This releases to the exhaust manifold of the engine the gas compressed in the cylinder and prevents the work done in compressing that gas from being recovered in the subsequent "power" stroke of the piston. The engine therefore dissipates the work of compressing the gas thus exhausted, and that dissipated work brakes the engine and the associated vehicle. In effect, the engine brake temporarily converts the engine to a gas compressor in order to make the engine absorb work or energy.
The typical compression release engine brake opens exhaust valves in the associated engine as described above by picking up an appropriately timed motion from another part of the engine and transmitting that motion to the exhaust valve or valves to be opened. For example, a fuel injector pushtube of one cylinder may be in motion when the engine brake needs to open the exhaust valve or valves of that cylinder or another cylinder. The engine brake may therefore be provided with a hydraulic circuit between a master piston operated by the fuel injector pushtube mechanism and a slave piston which operates on the exhaust valve mechanism. When the engine brake is turned on, this hydraulic circuit is sealed so that a fuel injector input to the master piston is transmitted to the slave piston, thereby causing the slave piston to open the associated exhaust valve or valves at the proper time.
If the engine associated with an engine brake is turbocharged, the force required from the engine brake to open the exhaust valves of the engine near top dead center of its compression strokes tends to increase substantially as engine speed increases and turbocharger effectiveness increases accordingly. A turbocharger uses the exhaust gases of the engine to spin a turbine. The turbine powers a rotary pump for forcing more gas into the intake manifold and cylinders of the engine. This increases the gas pressure in the cylinders, thereby increasing the force required to open the exhaust valves near top dead center of a compression stroke. A turbocharger has relatively little effect on the associated engine at low engine speed, but its effect increases substantially at higher engine speeds. When the engine is operating at high speed and the associated turbocharger is therefore highly effective, the force required from the engine brake in order to open the exhaust valves may become unacceptably high. For example, the load on the pushtube mechanism may exceed the load allowed on that mechanism by the engine manufacturer. This has typically necessitated adjusting the timing of the engine brake so that it does not attempt to open the exhaust valves so close to top dead center. However, this tends to reduce the effectiveness of the engine brake. Moreover, this reduction in engine brake effectiveness occurs at all engine speeds, even though the problem of unacceptably high loads or stresses only occurs at relatively high engine speeds.
Pearman et al. U.S. Pat. No. 4,688,384 shows pressure-sensitive relief valves for releasing gas from the intake manifold of a turbocharged engine when an engine brake associated with the engine is turned on and the intake manifold pressure exceeds the pressure required to open the relief valve. It has proven difficult, however, to produce systems of the type shown in the '384 patent that do not either suddenly and excessively drop the intake manifold pressure when the relief valve is opened, or that do not allow intake manifold pressure to continue to increase unacceptably even after the relief valve has begun to open. Commonly assigned, co-pending patent application Ser. No. 08/112,769, filed Aug. 26, 1993 shows apparatus for reducing turbocharger effectiveness during engine brake operation by somewhat restricting the flow of engine exhaust gas to or from the turbine of the turbocharger. However, this means placing moving parts in a high-temperature, particulate-laden environment.
In view of the foregoing, it is an object of this invention to provide compression release engine braking systems for turbocharged engines which provide increased engine brake effectiveness at low and moderate engine speeds without producing unacceptably high stresses in the engine and engine brake at high engine speeds.
It is another object of this invention to provide apparatus for smoothly and reliably reducing the increase in the pressure above a predetermined threshold pressure in the intake manifold of a turbocharged engine during operation of a compression release engine brake associated with the engine so that unacceptably high stresses in the engine and engine brake can be avoided at higher engine speeds.